A solar cell is a device which converts light energy to electrical energy. It is usually fabricated by forming an electrode layer, an absorption layer comprising a photo-electric converting semiconductor, and a light-transmitting electrode layer in this order on an electrical insulating substrate. Although a thin layer of the copper-indium-selenium ternary alloy having an atomic ratio of 1/1/2 is thought to have the highest photo-electric conversion efficiency when used as the photo-electric converting semiconductor layer, it has not always been easy to control the thickness of the alloy layer while controlling the proportions of the three components.
For forming such a thin alloy layer, vapor deposition (see, for example, JP-A-1-231313) and sputtering have been widely used. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, since these are film-forming techniques in which the three ingredients, i.e., copper, indium, and selenium, are successively or simultaneously deposited in a vacuum chamber, these methods have drawbacks, for example, that a large-scale apparatus is necessary for obtaining a homogeneous film having a large area, that the control of the vacuum system in the apparatus requires much time, and that it is difficult to control the composition and thickness of the film to be formed. Those two film-forming techniques are hence thought to be costly in film production and unsuitable for mass production.
An electroplating method in which copper and indium or an alloy of these is deposited in thin film form was proposed as a substitute for the vapor deposition or the sputtering. Furthermore, an improved method as a modification of the above electroplating method has been proposed, in which a copper-indium alloy layer containing selenium particles dispersed therein is formed using a copper-indium alloy electrodeposition bath containing fine selenium particles suspended therein (see International Publication WO 92/05586). However, these techniques also have various difficulties in controlling the composition of the electrodeposit to be formed.